In mobile communication systems, Massive MIMO (Multiple-Input Multiple-Output) attracts attention as technology of realizing high-speed and high-capacity transmission.
The Massive MIMO means multi-user MIMO transmission for transmitting a plurality of streams by using an extremely large number of element antennas. The Massive MIMO is implemented by, for example, installing in each base station an array antenna constituted by a large number of element antennas, and performing beam forming in addition to stream multiplexing.
The structure of digital massive MIMO, in which a digital to analog converter (DAC) being a digital-analog converter and other analog components are connected to all the element antennas and also a digital circuit is connected, is capable of obtaining a degree of freedom for the number of element antennas, which results in very high transmission performance.
However, the digital massive MIMO has problems in manufacturing cost and computational amount. Therefore, an antenna apparatus has been proposed, which reduces a circuit scale by forming an analog beam by using a phase shifter.
With regard to the antenna apparatus described above, a full-array system (for example, Non-Patent Literature 1), which refers to a method of forming a plurality of beams by using all element antennas, is capable of obtaining high antenna gain. However, in order to form the beams, a forming unit of the analog beam in this system may become complicated. Thus, there are problems in terms of manufacturing and cost.
On the other hand, in a sub-array system (for example, Non-Patent Literatures 2 and 3), which refers to a method using different element antennas for each beam, the structure of hardware is simpler than that of the full-array system, and arrangement of each sub-array and the like may be flexibly changed.
Patent Literature 1 mentioned below presents a proposal that a configuration to change size of antenna to be used (sub-array size) in dependence on a plurality of transmission modes including the MIMO.